In the art of fabricating tube and shell heat exchangers it has been known to create a joint between a tube end and its accommodating tube sheet or header plate by expanding the tube out into close fitting contact with the wall of a hole in which the tube end is received. This is frequently done by inserting into the tube end, after it has first been inserted in a hole in the header plate, a rotatable mandrel carrying rollers. In response to longitudinal penetration of the mandrel, and its simultaneous rotation, the tube end is radially deformed into close fitting contact with the hole wall.
In that area of the heat exchanger art which has seen development of the light weight, compact, high performance tubular heat exchanger, the use of mechanical tube joints has been abandoned in favor of metallurgical bonding, particularly brazing. In this process, one or both faces of the header plate is coated with braze alloy. After tubes have been installed in holes provided in the header plate or plates, the resulting assembly is heated to the melting temperature of the braze alloy and then allowed to cool. In the course of these steps, the braze alloy melts, flows to form fillets around the tubes, and, in hardening, forms a bond and a seal between the tubes and plate. Metallurgical bonding provides obvious benefits, including speed, uniformity and low cost, and is further valued for the positive seal it forms at the tube joints. In many compact, high performance heat exchangers, even minor leaks through the tube joints cannot be tolerated.
The brazed heat exchanger is not without disadvantages. Under some conditions of manufacture or of use, or of both, corrosion can become a problem. Very high pressures of circulated fluids, coupled with vibration and shock loading, can be destructive of tube joints. These considerations have led in some instances to demand for a non-brazed tubular heat exchanger, that is, a heat exchanger in which the tubes are joined to the header plates using only mechanical means. In attempting to comply with this demand, however, it has been discovered that conventional techniques of the past are inapplicable. Thus, among the steps taken to achieve light weight, high performance and compactness in tubular heat exchangers are steps to make the tubes of small diameter, on the order of one-eighth inch, to make the tubes of thin wall material, and to pack the tubes closely together for high density. These features increase the difficulties and risk involved in expanding tube ends into the header plate. The small diameter of the tubes rules out ordinary expansion techniques since no known expansion tool in the art is small enough to penetrate and work the tube interior.
It has accordingly been necessary to turn to innovative tools and practices in the effort to develop a practical method of mechanically joining tubes in header plates. In one such proposal, a rivet-like device or ferrule is placed on a slim, wire-like mandrel to limit against an enlarged head on the mandrel. The mandrel with mounted ferrule combination is inserted into a tube end in a header plate. A pneumatic gun or the like then pulls on the mandrel while applying an endwise reactant thrust to the ferrule. The result is to draw the enlarged head on the mandrel through the ferrule, expanding it and expanding also the surrounding tube end. This operation has been exhaustively tested, using many variations of tube, mandrel and ferrule size and materials, with and without a technique of counterboring the header plate to provide a space into which the tube end may expand. Even under controlled, laboratory conditions it has not been possible, using the foregoing method, to produce a leak free heat exchanger.
The foregoing discussion embodies a disclosure of all of the prior art of which we are aware, material to the question of patentability of the invention, and is intended as compliance with revised Section 1.97 of Title 37 of the Code Of Federal Regulations.